Spark ignited (SI) internal combustion (IC) engines, combustion turbines, and boilers operated with hydrocarbon fuels produce small amounts of undesirable chemical compounds in the combustion chamber, compounds which are exhausted from the engine at high temperatures (e.g., 800° F.-1250° F.). For fuels composed primarily of methane and other light hydrocarbons, the commonly regulated chemicals are nitrogen oxides (NO, NO2, or generally NOx) and carbon monoxide (CO). Nitrogen oxides are formed when nitrogen (N2), a major component of air, reacts with oxygen (O2), another major component of air, when both are exposed to high temperatures and pressures in an engine combustion chamber. Carbon monoxide, on the other hand, is the consequence of failure of the fuel to completely react with oxygen. Carbon dioxide (CO2) (and water) is formed when the fuel does react with oxygen.
CO and NOx are problematic pollutants inasmuch as their regulated values are in many geographical regions set at or below the limits of current technology. In addition, hydrocarbon emissions (CxHy), which result from the failure to completely react all of the fuel with oxygen, are also regulated.
It is known to use catalytic exhaust systems to reduce the emission of problematic pollutants from SI/IC engines. An example of such a catalytic system is a three-way catalytic converter (TWC). A TWC typically includes platinum as the active catalytic material on an alumina support.
Recently, multistage emission control systems have been proposed that include two or more stages of catalytic converters. An example of such a multistage system is disclosed in U.S. Pat. No. 8,578,704, entitled “Assembly and Method for Reducing Nitrogen Oxides, Carbon Monoxide and Hydrocarbons in Exhausts of Internal Combustion Engines,” assigned to the present Applicant, which is incorporated herein by reference. Multistage systems are used to improve the removal efficiency of pollutants by using multiple stages of catalytic converters, each configured to remove particular pollutants. For example, the '704 patent discloses a two-stage catalytic system where the exhaust is cooled (e.g., to less than 650° F.) and oxygenated between stages. The resulting system provides NOx removal in the first stage and CO and CxHy removal in the second stage.
A problem with multistage emission control systems is that the CO removal efficiency of the second stage catalytic converter can degrade over a relatively short time period, for example from an initial CO removal efficiency of 90% or more to a second CO removal efficiency of less than 20% after several hundred hours of operation.
Further, lean burn IC engines with single stage or multiple stage catalytic converters have particular difficulty with low temperature oxidation of CO and hydrocarbons (CxHy) as the catalyst quickly degrades after a relatively short time period of operation. For example, lean burn IC engines operating at temperatures below 650° F. can degrade from an initial CO removal efficiency of 90% to a second removal efficiency of less than 80% after several hundred hours of operation.
What is needed is a system and method that overcomes the above deficiencies.